1. Field of the Invention
The present invention relates to a fluid pressure monitoring assembly, and more particularly to a fluid pressure monitoring assembly including a pressure coupling mechanism for coupling a pressure transducer to a pressure transfer device with a biasing pressure established in the coupling mechanism between the pressure transfer device and the pressure transducer.
2. Description of the Prior Art
Heretofore various pressure monitoring assemblies have been proposed wherein pressure is transmitted through air to a pressure transducer. Examples of such assemblies are disclosed in the following U.S. patents:
______________________________________ U.S. PAT. NO. PATENTEE ______________________________________ 2,566,369 Putman 2,600,324 Rappaport 3,099,262 Bigliano 3,190,122 Edwards 3,364,749 Sipin 3,496,775 Sargent 3,537,357 Packer 3,738,356 Workman 3,831,588 Rindner 3,865,100 Kanai et al. 3,942,382 Hok 3,946,724 LaBalme ______________________________________
In the Workman U.S. Pat. No. 3,738,356 there is disclosed an intravenous pressure monitor which includes a flexible diaphragm secured over and about an opening leading to a needle. A dowel is situated between the diaphragm and a plate having a coil supported on the other side thereof about a magnet. Movement of the diaphragm is transmitted to the coil which moves relative to the magnet and generates an electrical signal which is used as an indication of pressure changes and causes a current to be applied to the coil to drive the membrane back to its original position.
The Rindner U.S. Pat. No. 3,831,588 discloses a pressure sensing device which includes an electrical pressure sensor mounted in a chamber on one side of a flexible membrane for transmitting external pressure to the sensor and a bellows is provided for varying the pressure in the chamber by known amounts thus enabling calibration of the pressure sensor device prior to use after the sensor has been mounted at its position in the chamber.
Also, heretofore it has been proposed in U.S. Application Ser. No. 843,223 for: MONITOR AND FLUID CIRCUIT ASSEMBLY, the disclosure of which is incorporated herein by reference, to monitor the pressure of liquid passing through tubings which are passed in and out of a centrifuge in a centrifugal liquid processing apparatus. When the liquid is blood which is being separated into components thereof, it is desirable that non-invasive pressure measurements be made and that those parts which are in contact with the blood be disposable to avoid cross-contamination. As described in prior application Ser. No. 843,223, a pressure monitoring assembly having a disposable pressure transfer device therein is coupled to a pressure transducer. The pressure transfer device is made of inexpensive disposable material and consists of a capsule divided by a flexible diaphragm into first and second chambers so that the liquid passing through the second chamber is monitored in a non-invasive manner. The first chamber is in communication with the pressure transducer and the air therein is at atmospheric pressure. It has been found that when the liquid pressure in the second chamber is much higher or lower than the air pressure, inaccurate monitoring results because of pressure loss over the diaphragm due to stretching of the diaphragm.
The pressure monitoring assembly and the coupling mechanism therein of the present invention differs from the assemblies referred to above by providing a coupling mechanism therein for establishing a biasing pressure in the pressure coupling mechanism greater than atmospheric pressure. As will be described in greater detail hereinafter, the biasing pressure is produced by compressing air trapped in the pressure coupling mechanism with a piston. By establishing a biasing pressure in the pressure coupling mechanism in the range of the liquid pressure being monitored, inaccuracies due to pressure loss across the diaphragm due to stretching of the diaphragm are minimized.